It is known to have intracardiac leads provided at their distal end with a screw for anchoring the distal head of the lead in the tissue of the endocardium so that an electrode of the lead makes electrical contact with the patient's myocardium tissue. In addition, in the case of a lead with an “active” screw, once in place the screw itself acts as a distal electrode for detection/stimulation of the myocardium. One such screw lead, with a retractable screw, is disclosed by EP0591053A and its counterpart U.S. Pat. No. 5,447,534 (both assigned to Sorin CRM S.A.S, previously known as ELA Medical), which describes a type of lead is marketed under the brand name STELIX (registered trademark) by Sorin CRM, Clamart, France.
These leads can be endocardial leads (i.e., placed in a cavity of the myocardium in contact with the wall thereof), epicardial leads (i.e., placed on the outside of the heart, in particular to define a reference potential, or to apply a shock), or intravascular leads (i.e., introduced into the coronary sinus to a location facing, e.g., the left ventricle wall).
The present invention is however applicable to other types of leads, for example, those leads having a distal electrode that remains on the surface of the wall of the patient's tissue to be detected/stimulated, and which are then provided with anchoring tines to maintain them in place at the chosen site. EP 0784993A1 (and its counterpart U.S. Pat. No. 5,800,499) and EP0779080A1 (all assigned to Sorin CRM S.A.S., previously known as ELA Medical) describe examples of such tined leads, which are incorporated herein by reference.
MRI examinations are presently contraindicated for patients with an implanted pacemaker or defibrillator. This is because of several problems caused by MRI:                Heating close to the electrodes of the lead connected to the generator;        Attraction forces and torques exerted on the device while immersed in the very high static magnetic field of the MRI device; and        Unpredictable behavior of the device itself, due to exposure to these extreme magnetic fields.        
The present invention aims to solve the first problem type. The heating problem appears especially in the vicinity of electrodes mounted at the distal end of the leads. Indeed, leads placed in the MRI imager act like antennas and pick up the radio frequency field (RF) emitted by the imager. Induced currents circulate in the conductors of the leads immersed in the RF field, causing heating of electrodes in contact with the blood and consequently heating of surrounding tissue. The heating at the electrodes is proportional to the density of current flowing therein and the smaller the surface of the electrode (the typical case being the surface of an active screw), the higher the current density and therefore the greater the heating of the surrounding tissue.
In practice, depending on the configuration of the generator, the leads and the MRI imaging, the temperature rise observed experimentally typically ranges from 8° C. (carbon electrodes) to 12° C. (metal electrodes), and sometimes even up to 30° C.
But the temperature increase should not exceed what is specified in the industry standard EN 45502-1 and its derivatives, which is less than 2° C. Indeed, an increase of 4° C. can cause a local cell death that has an immediate effect, among others, to substantially and irreversibly change the detection and stimulation thresholds, or even lead to complete loss of capture of the patient's heart beat.
It is certainly possible, as described in particular in U.S. Pat. Publication No. 2003/0204217 A1 and U.S. Pat. Publication No. 2007/0255332 A1, to provide an “MRI” safety mode in which a protection circuit connects to ground all conductors to prevent the flow of parasitic induced currents exposed to an MRI field. But this approach prevents the device from remaining functional for the duration of the MRI examination, which may last several minutes. It is therefore highly desirable that the implanted device can continue to provide seamless detection of depolarization potentials and possible delivery of stimulation pulses to the myocardium during an MRI examination.
To reduce the induced currents without disconnecting (i.e., open circuiting) or connecting to ground (i.e., grounding) the conductors, various techniques have been proposed, based primarily on putting in series with the conductor an impedance opposing current flow in an MRI examination situation. It may be a single coil (see, e.g., U.S. Pat. No. 7,123,013 B2), or a resonant tank circuit tuned to the RF frequency of the imager (see, e.g., U.S. Pat. Publication No. 2011/0106231, U.S. Pat. Publication No. 2010/0208397 A1 and U.S. Pat. Publication No. 2011/0054582 A1).
A passive protection circuit consisting in placing a PIN diode in parallel with a resistor in series with the electrode also has been proposed (cf. U.S. Pat. Publication No. 2008/0154348 A1). Yet another approach, disclosed in EP 2198917 A1 and its counterpart U.S. Pat. Publication No. 2010/0160989 (both assigned to Sorin CRM S.A.S., previously known as ELA Medical) is, in the case of a bipolar lead, to disconnect one of the conductors from normal functioning and to connect it to the ground of the housing of the generator, so that this conductor can act as protection shielding for the other conductor, which remains functional.
These options have, however, a number of limitations, including:                The systems implementing the commutations for their activation require a magnetic field detector in the generator;        Passive protection tank circuits are calculated for a specific imager frequency;        Integrated protection circuits may require an additional conductor in the lead and, consequently, cannot be used with any generator without suitable hardware modification of its circuits;        The protection components, such as diodes, are likely to cause substantial loss of energy when delivering stimulation pulses from the generator to the electrodes;        The addition of an integrated protection circuit in the distal portion of the lead body can cause, when the device is immersed in a MRI field, the reflection of an RF voltage to the generator that must be filtered and dissipated;        The incorporation of a protection circuit for self-protection of the lead in all circumstances is extremely sensitive in terms of technology, given the physical constraints: e.g., external diameter is limited to 5 French (1 F=⅓ mm), a need for an internal lumen in the lead body, and limitation of the length of the rigid part in the lead head; and        The continuing high cost of implementation of these technologies.        
Another disadvantage is that the protection circuit itself may undergo a rise in temperature that is transmitted to the proximal electrode and thus to the tissue of the heart wall.
Thus, in the U.S. Pat. Publication No. 2011/0106231 A1 above, the resonant tank circuit, which blocks the current flow to the anchoring screw forming an active electrode, comprises an inductor housed in the lead head. When the assembly is placed in a MRI field at a frequency corresponding to that of the tank circuit, high current flows in the inductor and causes significant heating within the lead head. The document proposes to evacuate this heat to the outside by equipping the lead head with a liner in the outer region of the inductance, forming a heat radiator. The heat produced in the inductance is then distributed radially in the lead head and then through the liner, to transfer the heat to the surrounding blood flow.
It is emphasized that in this structure the thermal diffuser is placed in line with the inductor (that is to say at about the middle of the terminal part of the lead head), and that the distal end of the lead is not affected by the thermal diffuser. Specifically, the distal end is made of a flexible material such as silicone which limits the contact pressure on the tissues. Silicone is a poor conductor of heat, but this is not a problem because in the presence of an RF field the screw is no longer powered (due to the tank circuit) and the tissues are not warming up at this level.
It should be understood that the structure described in this document requires the presence of a relatively large inductance, and electrical isolation between the inductor (which is connected in series with the stimulation conductor) and the thermal diffuser (which is necessarily in contact with the surrounding blood medium). All these constraints increase the complexity of implementation and the overall volume of the lead head.
U.S. Publication No. 2010/0208397 A1 and U.S. Pat. Publication No. 2011/0054582 A1 described above, disclose comparable lead head configurations, with a tank circuit to prevent against the harmful effects of an MRI field, and a heat diffuser arranged in line with the inductance of the tank circuit, so as to allow transfer of the heat generated in the inductor in the radial direction to the surrounding blood medium.